This invention relates to a new adhesive coating composition, particularly for use on airbag fabrics, which is composed of a crosslinked blend of a polyalkyl- or polyphenylsiloxane with vinyl functionality and a copolymer of ethylene and methyl acrylate. This coating provides excellent adhesive properties to the coated fabric, in that it restricts the movement, when put under stress, of the yams constructing that fabric, which are situated adjacent to seams in the fabric. This reduces the effect known in the trade as combing. Fabric treated with this material in the manner described herein, requires a marked increase in the amount of force required for this yarn movement. This distortion of the fabric can be detrimental in an automotive airbag, in that, it may result in the loss of inflating air, and under extreme conditions may result in a seam failure.
Airbags for motor vehicles are known and have been used for a substantial period of time. These devices are installed on the driver and passenger side of automobiles and, in the event of a collision, are rapidly inflated with gas, to act as a barrier between the driver or passenger and the steering wheel or dashboard of the automobile.
Coatings have been applied to fabrics, intended for use in automotive airbags, to resist the unwanted permeation of air through the fabric and, to a lesser extent, to protect the fabric from detriment by the hot gases used to inflate the bags. Polychloroprene was the polymer of choice in the early development of this product, but the desire to decrease the folded size of the completed airbag, and the tendency of polychloroprene to degrade, with exposure to heat, to release the components of hydrochloric acid (thereby potentially degrading the fabric component as well as releasing hazardous chemicals), has led to the almost universal acceptance of silicone (polydimethylsiloxane or similar materials) as a more suitable coating. In the quest for the most compact folded size possible, coating levels of polymer have dropped from around 2.5 ounces per square yard of fabric, to levels approaching 0.5 ounces per square yard.
New developments in airbags, particularly newer designs being placed in the sides of the passenger compartment, have introduced the requirement that the bags hold pressure longer under use. This, and the evolution of the lower coating levels of silicone polymer, have begun to highlight the effect that, when a sewn seam is put under stress, a naturally lubricating silicone coating may allow the yarns from which the fabric is constructed to shift. This shifting can lead to leakage of the inflating gas through the new pores formed from the shifting yams, or, in drastic cases, cause the seam to fail. Since the airbag must retain its integrity during a collision event, in order to sufficiently protect the driver or passenger, there is a great need to provide coatings which provide both effective permeability characteristics and sufficient restriction of yarn shifting for the airbag to function properly, if and when necessary. Therefore, a need exists for a cost-effective, one-layer coating which provides low permeability, resistance to yarn shifting and age resistance over long periods of storage.
It is therefore an object of the invention to provide an adhesive coating composition for fabric substrates, which provides substantial reduction of the tendency of yarns, located at cut edges or seams in the fabric, to shift when put under stress. It is a further object of the invention, to provide a highly cost-effective, easily handled and simple-to-make formulation for use as an adhesive coating on fabric surfaces. Yet another object of the invention is to provide a highly effective coating for airbag fabrics, bags and cushions, which substantially reduces the chances of an airbag failure due to bursting or torn seams, during inflation initiated by a collision event.
The inventive coating (which may be utilized on any fabric substrate) provides such an improvement. This coating is comprised of a polysiloxane polymer with vinyl functionality, blended with an ethylene-containing copolymer (in combination with a substituent which has a polar nature) the entire system being cured with an agent capable of crosslinking both of these polymers. More specifically, the inventive material is comprised of a polyalkyl- or polyphenylsiloxane, blended with a copolymer of ethylene and, as one non-limiting preferred substituent, methyl acrylate or a copolymer of ethylene and, as another non-limiting preferred substituent, vinyl acetate, and cured with a peroxide. It has been found that the ethylene-methyl acrylate copolymer provides the most desired adhesive qualities upon incorporation with the vinyl-containing polysiloxane, while retaining the other required properties for an airbag fabric.
Preferably, the coating is produced by combining, in solution, 50-95 parts of the polysiloxane polymer, 5-50 parts of the ethylene-containing copolymer and 2-10 parts of an organic peroxide curing agent. The siloxane component may be comprised of a polyalkylsiloxane, a polyarylsiloxane or a copolymer of both, each containing at least one vinyl functionality to allow appropriate crosslinking. Specifically, the preferred polyalkylsiloxane is a vinyl-containing polydimethylsiloxane and the preferred polyarylsiloxane is a vinyl-containing polyphenylsiloxane, either of these materials may contain nondisclosed materials common to the compounding of commercial materials of this sort. The ethylene-containing copolymer may be a copolymer comprised of 60-85 weight percent ethylene and 15-40 weight percent methyl acrylate, preferably from 65-75 weight percent ethylene and 25-35 weight percent methyl acrylate, and most preferably a distribution of about 66-67 to about 33-34 of the two components. It may also be an ethylene containing copolymer comprised of 20-60 weight percent ethylene and 40-80 weight percent vinyl acetate, preferably from about 25-50 weight percent ethylene and from about 50-75 weight percent vinyl acetate, and most preferably about 40 and about 60 weight percent of these two components. The preferred curing agent is an acyl peroxide, typically, but not limited to, benzoyl peroxide. Other well known curing agents may also used be used either supplemental to or as a substitute for the preferred peroxide. Also, a solvent is preferably present to effectuate proper and thorough mixing of the components of the preferred coating composition. More preferably such solvent is organic and volatile (i.e., evaporates easily at a relatively low temperature); most preferably such a solvent is toluene or similar type volatile liquid.
The coating may contain silane coupling agents of the type well known to the trade for improving the adhesion of silicone containing coatings to fabrics (such as trimethoxysilanes, as merely a broad example). It may also contain pigments or colorants, (for identification or aesthetic reasons); inert ingredients (such as calcium carbonate or other materials classified as fillers), to reduce cost without also reducing the desired performance of the inventive coating as applied to the target fabric; flame retardants; and processing aids necessary to process the composition and make it suitable for use as an airbag coating.
This inventive coating composition may be applied to any type of fabric substrate to alleviate seam tears and unraveling at cut edges. Thus any type of fabric may be utilized in conjunction with the inventive coating, including those comprising natural fibers, such as cotton, ramie, abaca, wool and the like; synthetic fibers, such as polyester, polyamide, regenerated cellulose and the like; and inorganic fibers, such as glass, boron derivative fibers and the like. Furthermore, the target fabrics may be woven, knitted, non-woven and the like. Preferably, the target fabric substrate comprises synthetic fibers, preferably, polyesters and polyamides and is preferably an airbag fabric.
If an airbag fabric is desired, any standard low permeability airbag fabric construction may be utilized as the target airbag fabric with the inventive composition. Preferably, however the target airbag fabric within this invention is a plain woven fabric, formed from yarns comprising polyamide or polyester fibers. Such yarn preferably has a linear density of about 210 denier to about 630 denier. Such yarns are preferably formed from multiple filaments, wherein the filaments have linear densities of about 6 denier per filament or less and most preferably about 4 denier per filament or less. Such substrate fabrics are preferably woven using fluid jet weaving machines as disclosed in U.S. Pat. Nos. 5,503,197 and 5,421,378 to Bower et. al. (incorporated herein by reference). The fabric substrate with applied coating will hereinafter be referred to as an airbag base fabric.
In accordance with the potentially preferred practices of the present invention, a toluene dispersion comprised of from 20-30 percent non-volatile materials, is constructed from the materials described above. The potentially preferred dispersion comprises 65 phr of the vinyl functional polysiloxane, available under the tradename SilasticÔ HS-30 silicone rubber from Dow Corning (which is believed to have a place of business at Midland, Mich.), 35 phr of the ethylene-methyl acrylate copolymer, available under the tradename Vamac DÔ from DuPont Polymers (which is believed to have a place of business at Wilmington, Del.), 5 phr of benzoyl peroxide and 1 phr of 3-methacryloxypropyltrimethoxysilane available under the tradename Silane Z-6030Ô from Dow Corning.
Once compounding is complete, the formulation is preferably scrape coated across the airbag base fabric, the solvent (toluene, for example) is subsequently removed by evaporation by placing the treated airbag base fabric in an oven at 60-90xc2x0 C., and the resultant coated airbag base fabric is then cured in an oven at 150-200xc2x0 C. to form a thin coating. Scrape coating, in this sense includes, and is not limited to, knife coating, in particular, knife over roll, knife over gap, knife over table, floating knife and knife over foam pad methods, to name a few method types. Since the coating composition exhibits excellent adhesive properties, generally only one coating pass is necessary to provide an effective, stable, low permeability coating on the target fabric surface.
The final dry weight of the coating is preferably about 2 ounces per square yard or less and most preferably 1 ounce per square yard or less. The resultant base fabric is also substantially impermeable to air, when measured according to ASTM Test D737, xe2x80x9cAir Permeability of Textile Fabricsxe2x80x9d.
As previously indicated, the substrate fabric is preferably a woven nylon material. In the most preferred embodiment, such substrate fabric will be formed from fibers of nylon 6,6. It has been found that such polyamide materials exhibit particularly good adhesion when used in combination with the coating according to the present invention.